Sources : Onocentaur

Aelianus [170-230 CE] (On the Characteristics of Animals, Book 17, chapter 9): There is a certain creature which they call an Onocentaur, and anybody who has seen one would never have doubted that the race of Centaurs once existed, and that artificers did not falsify Nature, but that time produced even these creatures by blending dissimilar bodies into one. ... Its face is like that of a man and is surrounded by thick hair. Its neck below its face, and its chest are also those of a man, but its teats are swelling and stand out on the breast; its shoulders, arms, and forearms, its hands too [and its] chest down to the waist are also those of a man. But its spine, ribs, belly, and hind legs closely resemble those of an ass; likewise its color is ashen, although beneath the flanks it inclines to white. - [Scholfield translation]

Isidore of Seville [7th century CE] (Etymologies, Book 11, 3:39): The Onocentaur is so called because it seems to look half like a human, half like an ass. Likewise the Hippocentaur, which is thought to combine the natures of horse and human in itself. - [Barney, Lewis, et. al. translation]

Thomas of Cantimpré [circa 1200-1272 CE] Liber de natura rerum, Quadrupeds 4.82): The onocentaur, as Isidore and Andelmus say, is a monstrous animal, and of two forms by nature. For it has a head like that of an ass, and his body like that of a man. ... Others, however, say that the onocentaur had the body of a horse, but the upper part of a man [centaur]. It has a hideous face and hands capable of any action. Onocentaurs sound as if they are beginning to speak as they uttered their voice, but their unaccustomed lips can not form a human voice. Andelmus the philosopher says that this monster was not created as such from the beginning of creation among other beasts, but wherever and whenever it is seen in any part of the world, it is born from the adulterous mixture of a man and a bull, or a man and a horse. But many contradict this opinion. Blessed Anthony saw this monstrous animal as he continued to mourn in the desert, as blessed Jerome writes. The glorious Paulus the first hermit asserts a doubt whether the devil invented this or whether nature created this. This monstrous animal was in the region of the East, and was naturally created, like many other monsters, as worthy and due to the admiration of the founder. After those who pursue him, as the Experimentator says, it sends sticks or stones. - [Badke translation/paraphrase]

Bartholomaeus Anglicus [13th century CE] (Liber de proprietatibus rerum, Book18.78): Onocentaurus, as the Glose sayth super Esay. 9. is a beast of a straunge forme, and is gendered betwéene a Bull and an Asse. For an Asse is called Onos in Gréek: and so it is a beast lecherous as an Asse, and stronge necked and nowled as a Bull. But Phisiologus meaneth otherwise, and sayeth. That Onecentaurus is compounded of the shape of a man and of an asse: for he hath shape of a beast from the navell downwarde. It séemeth that Plinius accordeth héereto libro. 7. cap. 3. where he sayeth, that wise and wittie kinde maketh to us gameful things and wonderfull, to shew his might. And in the same chapter hée setteth example of many wonderful shapen beasts, which be in Indie, as of Faunie and Satiris and Onocentauris, and of other such, which hee calleth beastes, and feineth somwhat the shape of mans kinde. And other meane, that Centauri were called Horse men of the Countrye of Thesalon, which pricked up & downe on horses, and therfore some of them séemed that horse and man were one bodye, and so they accounted, that Centauri were then feyned, as he sayth, lib. 11. where he speaketh of beastes wonderfullye shapen: and Centaurus in Gréek, and this name Centaurus is compounded with Onos and Centaurus, and so Onocentaurus hath that name, for the halfe thereof hath the shape of a man, and halfe of an Asse, as Ipocentaurus is a beast wonderfullye shapen, in whome is accounted the kind of man and of an horse, as Isidore saith. - [Batman]